This invention relates to suspensions of isocyanato ureas in isocyanate prepolymers; to a process for their production by reacting mixtures of aromatic diisocyanates containing isocyanate groups of different reactivities and prepolymers containing terminal isocyanate groups, with water; and to the use of the suspensions as synthesis components in the production of polyurethane plastics.
It has long been known that the reaction of water with monoisocyanates gives substituted ureas and, with polyisocyanates, gives polyureas of high molecular weight. According to U.S. Pat. No. 2,597,025, resin-like polyureas containing isocyanate groups can be obtained by using from 0.3 mol of H.sub.2 O per mol of aromatic diisocyanate in suitable solvents. In addition, it is known that aromatic diisocyanates can also be selectively reacted with water to form low molecular weight diisocyanato ureas. Thus, according to U.S. Pat. Nos. 2,757,184, 2,757,185 and 3,906,019, the reaction of 2,4-diisocyanatotuluene with water under suitable conditions gives the corresponding bis-(3-isocyanatotolyl)-urea.
The analogous reaction of 2,6-diisocyanatotoluene to form the corresponding 1,3-bis-(3-isocyanatotolyl)-urea is also known (U.S. Pat. Nos. 3,906,019 and 2,902,474).
All of these processes are carried out in solvents. The isocyanates must be highly soluble. The water added must be at least partly soluble in the solvents used. The solvent must not have any polymerizing effect on the isocyanate and must be free from isocyanate-reactive functional groups. The isocyanate ureas are precipitated in the form of substantially insoluble compounds and are isolated by filtration. For further processing for polyurethane reactions, these isocyanate ureas obtained by filtration and freed from the solvents in vacuo must be converted into a finely divided form by grinding operations. Due to the high melting point and to the poor solubility of these isocyanate ureas, inhomogeneous products are obtained in many cases during the further reaction.
Isocyanato ureas, particularly bis-isocyanato ureas of low molecular weight, are particularly valuable starting materials for the production of polyurethane plastics because the urea groups incorporated during their use provide the plastics with very good mechanical properties.
Accordingly, the object of the present invention is to provide bis-isocyanato ureas by a simple process and in a form which enables them to be further processed without difficulty into high molecular weight polyurethanes.